Abstract
1. Thin diaphragm muscles in dialysed serum maintained their total sodium at values similar to those found in vivo. The fibre sodium exchanged with a half-time of 5 min, and the flux was 10 p-mole.cm(-2).sec(-1). The internal sodium was less than 10 mumoles/g fibre water and the ratio of external to internal sodium was at least 15. The calculated energy expenditure for sodium extrusion was less than 2% of the resting metabolism.2. In physiological saline the half-time for exchange of fibre sodium was similar to that in serum, but in saline there was an increase in sodium permeability and a raised total and fibre sodium.3. Muscles treated with insulin (0.02 u./ml.) showed a more rapid exchange of sodium compared with muscles in saline, with no change in the permeability to sodium. Insulin also affected sodium movements in the absence of external glucose.4. In saline the fraction of sodium which exchanged rapidly occupied 0.33 by volume of the muscle, after corrections for diffusion, and this value was similar to the mannitol space.5. Fibre diameters were measured in frozen sections. Muscles prepared by fixation and embedding showed marked shrinkage.